1. Field of the Invention
The present invention relates to an improved method and apparatus for controlling particulate emissions from coal-fired boilers and the like, and, more particularly, to the use of an improved system for extracting a side-stream flow of fine-particulate-laden effluent gases from a mechanical separator so that the side-stream flow can be treated to remove particulate emissions.
2. Prior Art
Large, stoker-fed, coal-fired boilers are used in commercial and industrial installations to provide heat for generation of electricity, for certain industrial processes which require high heat input, and for the heating of buildings. Exhaust gases from such boilers contain a sufficiently high quantity of particulate matter to require treatment in order to comply with air pollution control regulations.
Essentially three types of exhaust gas treatment units have been employed to remove particulate emissions from exhaust gases, namely mechanical collectors, electrostatic precipitators and fabric filters. Of these three types of separation equipment, mechanical separators are found to be less expensive to install and operate than are the other alternatives. High efficiency mechanical collectors have been developed which have proved to represent the most reliable and cost-effective manner for controlling particulate emissions from large stoker-fed, coal-fired boilers.
Where boilers and their associated mechanical collectors are maintained and operated properly, approximately 96 percent of the particulate emissions from exhaust gases can be removed with mechanical collectors at a cost of about $2.00 per pound of steam. If particulate emissions must be reduced still further, relatively high costs have been incurred in using currently developed technology. By way of example, fabric filter equipment can remove another 2 or 3 percent of the particulate matter, but at a cost of about $12.00 per pound of steam, or about 6 times the cost of operation of mechanical collectors.
A typical mechanical collector includes a housing which encloses a cluster of upstanding collector tubes through which effluent gas is passed in a downward direction. Each of the collector tubes includes an outer tube which has a tapered, funnel-shaped, lower end portion, and an inner tube extending into the upper end region of the outer tube. In operation, effluent gas enters a collector tube by passing between its inner and outer tubes. The gas is caused to take on a rotating, cyclonic motion by a series of vanes deployed between the inner and outer tubes.
The cyclonic motion of the downwardly moving gas creates a vortex. The centrifugal force of the vortex forces particulate matter outward against the inside wall of the outer tube, where it travels downwardly and falls, under the influence of gravity, into a hopper located beneath the lower end of the outer tube. The cleaned, swirling gas is tightened into a smaller vortex by the gradually tapering configuration of the lower end of the outer tube. At a location near the bottom of the outer tube, the clean gas reaches a point of equilibrium, reverses direction, and passes upwardly through the center of the inner tube under the influence of an induced draft fan. This clean gas is then ducted to a stack for discharge into the atmosphere.
The described mechanical collector works quite effectively to remove particulate matter greater than 10 microns in size. However, where very fine particulates are entrained in effluent gases, the particles (typically having a size less than 10 microns) are less susceptible to centrifugal forces, and a larger proportion remain suspended in the cleaned exhaust gases. Moreover, the sudden reversal in direction of gas flow which occurs near the bottom of each collector tube tends to re-entrain some of the fine particles that were already separated and would otherwise have remained separated.
In an effort to improve the operation of a mechanical collector of the type described above, it has been proposed that a portion of the gas stream which is heavily laden with fine particulate matter be drawn off and passed through a relatively small fabric filter unit. The proposal which has been made postulates that the drawing off of such a flow, which has come to be known in the art as a "side stream," would result in a relatively small quantity of fine-particulate-laden gas to treat. The proposal further postulates that the drawing off of a side stream flow would produce an additional downward "pull" through the collecting tube array for the remaining 80 to 90 percent of the gas stream, thereby reducing the tendency toward re-entrainment of fine particles which have already been separated. Thus the operation of a mechanical collector would be significantly enhanced, and the need for additional expensive equipment to filter fine particulates from the entire flow of cleaned gases would be eliminated.
The type of proposal which has been made for implementing and testing the side stream separator theory involves only minor changes to a conventional mechanical separator. An insulated ductwork is connected to the outer housing of the separator at a position immediately below the location of what is called the "collecting tube sheet," i.e., a metal plate which extends across the inside of the collector housing, and through which the upper ends of the collector tubes pass to receive effluent gases. Relatively large holes are formed through one or more sides of the collector housing to communicate the ductwork with the inside of the housing at a position below the location of the collecting tube sheet. An induced draft fan is coupled to the ductwork and serves to draw a side stream flow from within the housing through the ductwork and through a conventional fabric filter unit where particulates are separated from the side stream gas flow.
The above-described proposal includes no modifications to the mechanical separator itself, other than to form relatively large holes in the sides of its housing to communicate the ductwork with the inside of the housing. While the proposal includes the observation that the theoretically optimum locations for the take-off ducts leading from the mechanical collector would probably be "at each individual collection tube," the proposal discards these theoretically optimum locations as being impractical to achieve due to the complexity of the system that would be needed in order to serve the large number of tubes utilized in each mechanical collector. The proposal concludes that the best practical alternative is to simply attach the ductwork to the sides of a mechanical collector and to establish entry communication through relatively large holes formed in the sides of the collector.